This invention generally relates to sensors. More particularly, it relates to variable reluctance transducers. Even more particularly, it relates to an improved system for greater accuracy in determining position or displacement in view of changes in temperature.
Sensors are being developed for use in roads, bridges, dams, buildings, towers, and vehicles that may provide many information, including displacement, strain, speed, acceleration, temperature, pressure, and force.
Temperature changes can effect components within a sensor, and one challenge has been that sensor output has varied with temperature of sensor components. The effect of temperature on inductive transducers limits their overall absolute accuracy. To reduce the temperature effect, inductive transducers, such as a differential variable reluctance transformer (DVRT), have been designed with a differential pair of coils. A DVRT includes a ferrite core that moves within two coils that are arranged in a bridge configuration. Inductance and impedance of the coils changes with displacement of the ferrite core within them, and the displacement of the core is accurately determined by the change in inductance or impedance of the coils. Since the output signal of a differential pair is the difference between the output of two coils, temperature changes that both coils experience equally are theoretically subtracted out. However, if one coil experiences a different temperature environment than the other coil, a signal proportional to the temperature gradient between the two coils will appear at the circuit output, significantly reducing absolute accuracy.
Commonly assigned U.S. Pat. No. 5,914,593 (xe2x80x9cthe ""593 patentxe2x80x9d), incorporated herein by reference, provides a circuit for a DVRT that compensates for a temperature gradient across a sensor. The ""593 patent recognizes that a temperature gradient across the coils can provide a change in the resistance of the wire forming one of the coils more than it changes the resistance in the other coil, and this difference in resistance can change the differential impedance of the coils, essentially mimicking a change in position, resulting in an error in measured displacement. Situations where one coil may be hotter than the other coil are common in applications such as automotive. In the ""593 patent a circuit is provided to adjust the output compensating for the difference in resistance in the two coils introduced by a temperature gradient or a temperature difference between the coils.
However, the present inventors found that not all temperature effects were compensated by the circuit of the ""593 patent. Thus, a better system for acquiring data is needed that provides improved temperature compensation, and this solution is provided by the following invention.
It is therefore an object of the present invention to provide a circuit that improves compensation for temperature changes in a sensor;
It is a further object of the present invention to provide a circuit that corrects for uniform temperature change over time and for spacial gradients in temperature;
It is a feature of the present invention to provide an ac signal and a dc signal to a differential sensor in a Wheatstone bridge configuration and to correct for time and spacial temperature differences based on the ac and dc output signals;
It is a feature of the present invention to provide a time varying temperature compensating circuit in combination with a temperature gradient compensating circuit;
It is an advantage of the present invention that more accurate data can be collected from a DVRT that is independent of variations in temperature either in time or space.
These and other objects, features, and advantages of the invention are accomplished by an electronic device that includes a sensor having a magnetically permeable member and a circuit. The circuit adjusts sensor output to provide sensor output data independent of temperature of the magnetically permeable member. The circuit uses a signal derived from resistance of the sensor to correct for temperature.
Another aspect of the invention is accomplished by an electronic device that includes a coil, a magnetically permeable member that extends in the coil, and a circuit. The circuit adjusts output voltage of the coil to compensate for a change in temperature in the coil and in the member.
Another aspect of the invention is accomplished by an electronic device that includes an inductor, a magnetically permeable member coupled to the inductor, and a circuit. The circuit adjusts a voltage output of the inductor to provide a voltage independent of temperature of the inductor and temperature of the magnetically permeable member.
Another aspect of the invention is accomplished by an electronic device for sensing at least one parameter that includes a first circuit element comprising a reactance and a resistance, the first circuit element comprising input terminals and output terminals. The input terminals are for providing a first input signal and a second input signal different from the first signal to the first circuit element. The output terminals are for providing a first output signal and a second output signal from the first circuit element. A second circuit element is connected to the output terminals to use the first output signal and the second output signal, wherein the second circuit element generates a first parameter that depends exclusively on the resistance and a second parameter that depends exclusively on the reactance. A third circuit element is connected to the second circuit element wherein the third circuit element compensates the second parameter for changes in the first parameter.
Another aspect of the invention is accomplished by a sensor comprising a component and a circuit. The component is used by the circuit both for sensing a first parameter and for sensing temperature. The temperature is used in the circuit for correcting the first parameter to make output of the sensor independent of change in temperature with time.